The interturbine duct (ITD), sometimes referred to as the interstage duct, channels hot combustion gases from an axial high pressure turbine (HPT) stage to an axial low pressure turbine (LPT) stage. In multi-spool turbofan engines, the ITD is an annular duct of significant length which is typically cast integrally as a part of the LPT vane set, and thus forms in essence an extension of the LPT vane, as shown in U.S. Pat. No. 5,485,717. As gas turbine engine size decreases, the casting size becomes an increasing proportion of the engine weight, since castings cannot scale down linearly as castings can only be made reliably down to a certain minimum thickness. U.S. Pat. No. 5,016,436 discloses a double-skinned sheet metal ITD arrangement, in which cooling air is circulated between the skins to cool the hot inner skin. The double skin also provides stiffening against the dynamic forces which the ITD encounters in normal use. Such a configuration is complex and bulky, however, not to mention expensive to manufacture.
Accordingly, there is a need to provide a new lightweight ITD construction.